Conventionally, to improve the strength and like properties of resin, use of a filler to reinforce resin has been known. In particular, glass fiber is used in various fields because of its excellent mechanical properties. However, although glass fiber has excellent mechanical properties, it has problems in that the resulting molding material becomes heavy due to high relative density, and a large amount of residue is generated at the time of disposal. On the other hand, it has been known that a microfibrillated plant fiber (nanofiber) that is refined to a nano-order fiber diameter size by microfibrillating a cellulose fiber obtained from wood or a herb is lightweight and has high strength. Recently, attempts have been made to obtain a lightweight resin molding material having high strength by using such a microfibrillated plant fiber.
For example, Patent Literature 1 discloses a method in which pulp and/or cellulose-based fiber that has been subjected to a simple pretreatment in which a primary wall outer layer and a secondary wall outer layer are damaged is melted and kneaded with a resin component in the presence of a swelling agent of cellulose amorphous regions to defibrate fiber components during melting and kneading, thereby microfibrilating the fiber components, and uniformly and finely dispersing the fiber components in the resin component.
Patent Literature 2 discloses a material with high strength comprising 65 to 100% by weight (solids content) of a cellulose microfibril and 0 to 35% by weight of an additive. Additionally, as methods for microfibrilating pulp to obtain microfibrils, Patent Literature 2 discloses methods for treating pulp, such as treatment using a medium stirring mill, treatment using a vibration mill, treatment using a high-pressure homogenizer, and treatment using a stone mill. In addition, as additives, Patent Literature 2 discloses thermosetting resins such as phenol resin, urea resin, melamine resin, unsaturated polyester resin, epoxy resin, diallyl phthalate resin, polyurethane resin, silicone resin, and polyimide resin.
Patent Literature 3 discloses conventional fiber reinforcing plastics that use as a reinforcing material, a lignocellulose fiber, in which the cell wall of the fiber is transformed to lose intracellular space, which is a hollow portion.
Patent Literature 4 discloses microfibrillating a cellulose fiber having a specific length using a homogenizer (high-pressure homogenizer, etc.) to obtain a microfibrillated cellulose having excellent water retentivity and long fiber length despite a small fiber diameter. However, Patent Literature 4 relates to a microfibrillated cellulose useful for special paper and filtration material requiring strength, to a method for producing the same, and to a nonwoven sheet comprising the microfibrillated cellulose; Patent Literature 4 does not teach a composite with resin.
In Patent Literature 1 to 4, since microfibrillated plant fibers are obtained by a mechanical treatment such as a treatment using a twin-screw kneader or a stone mill, treatment using a PFI mill, or treatment using a high-pressure homogenizer, a large amount of energy is required to microfibrillate all of the plant fibers, and fiber breakage occurs during the microfibrillation. Therefore, the intrinsic properties of microfibrillated plant fibers have not been fully developed.
By combining a chemical treatment with a mechanical treatment, attempts to perform nanofibrillation have been made while reducing damage such as breakage by not applying an excessive shearing force.
For example, Patent Literature 5 discloses that an oxyl compound such as 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) is reacted with a natural cellulose material using a co-oxidizer to oxidize a part of the C6 primary hydroxyl groups in a cellulose to aldehydes or carboxy groups by using aldehydes, thus obtaining refined cellulose fibers having a number average diameter of 150 nm or less by a relatively light mechanical treatment, i.e., by electrostatic repulsion. Although Patent Literature 5 discloses that the invention can be applied as a nano-filler for a composite material, there is no specific description of forming a composite material with resin. Additionally, Example 1 teaches that the obtained nanofiber dispersion has a content of 0.1% by weight, and is transparent and slightly viscous. Thus, there are various problems such that a large amount of energy is required for dehydration to form a resin composite, and modified aldehyde groups or carboxy groups easily cause heat deterioration of microfibrillated plant fibers in the formation of a thermosetting resin in the resin mixing step.
Further, Patent Literature 6 discloses a method for semi-esterifying some of the hydroxyl groups by a polybasic acid anhydride. In the cellulose in which a part of the hydroxyl groups are semi-esterified by a polybasic acid anhydride, the modified portion has ester bonding; therefore, the modified portion problematically causes a side reaction such as hydrolysis. Thus, when such modified cellulose fibers are used as a resin molding material, further improvement in strength can be expected.
Furthermore, Patent Literature 7 discloses a cellulose derivative whose water retentivity and re-dispersibility in water are improved, by subjecting cellulose fibers to carboxymethyl etherification, and then microfibrillation; however, mechanical pulverization or grinding, which is a method for microfibrillating cellulose fibers, is performed by a dry method or a wet method that uses a non-swelling solvent as a medium. Therefore, although a material having good water retentivity and water dispersibility can be obtained, the cellulose derivative has room for improvement when used as a reinforcer for a resin molding material because of insufficient nanofibrillation of plant fibers.